Modern electronic circuitry is generally mounted onto printed-circuit boards (PCBs), also termed printed wiring boards (PWBs), using surface or through-hole techniques. Typically, copper patterns are formed on substrates by etching or plating techniques or used in multi-layer structures composed of stiffeners such as woven fiberglass and epoxy, although various alternative materials are also in use both for the conductors and dielectric supporting materials involved.
Due to the desire for higher levels of integration and functionality, such boards have, in some cases, become exceedingly complex, and may incorporate 20 layers or more with conductors and/or spaces of less than 5 mils, including buried and blind vias to facilitate connections between levels internal to the final sandwich-like structure.
As with many electronic assembly techniques, as difficult as it is to define and implement interconnect patterns, and so forth, it is even more difficult to repair circuits found to be substandard. Often PWBs with even minor problems are better off being discarded, resulting in a considerable loss to the manufacturer. Although open and Short circuits may be easy to locate, more insidious problems occur when the conductor "nets" are properly defined, but, as a result of processing deficiencies, exhibit electrical characteristics, such as resistance, which are not within design specifications. Such problems may lead to boards which function properly for a while, then fail, or develop improper behavioral characteristics as a result of environmental conditions such as temperature and humidity variations.
Such properly-routed but out-of-specification electrical characteristics can be particularly difficult to analyze and resolve, not only because the circuits themselves and board patterns must be initially treated with suspicion in light of unusual or erratic behavior, but once products are shipped, failure analysis becomes much more difficult, and more expensive, since the process often involves dismantling of the equipment to facilitate debugging, and usually such tests are performed by individuals different from those who designed or built the subsystems, and may therefore be less familiar with it.
To further complicate matters, even if it is determined that the PWB is at fault, as opposed to other electrical connections, components or integrated circuits involved, many different pieces of processing equipment and fabrication steps are often involved in the conductor formation process, and each may need to be investigated in order to find the source of problems. For example, problems may lie with raw materials, the imaging systems used to expose the desired patterns, the photoresist used to transfer the image, the developing and etching equipment, and the chemicals used in the equipment, and so forth.
The industry will surely benefit, then, from a system and/or techniques useful in characterizing conductor formation processes, whether for printed circuit or wiring boards, multi-chip modules, flat-panel displays or other applications, thereby enabling a manufacturer to know, in advance, where potential problems lie, further allowing such problems to be tracked down and corrected prior to final product assembly.